Wilson D, Valluzzi R, Kaplan D
Department of Chemical Engineering and Biotechnology Center, Tufts University, Medford, Massachusetts 02155 USA.
Biophys J. 2000 May;78(5):2690-701. doi: 10.1016/S0006-3495(00)76813-5.
Protein structural transitions and beta-sheet formation are a common problem both in vivo and in vitro and are of critical relevance in disparate areas such as protein processing and beta-amyloid and prion behavior. Silks provide a "databank" of well-characterized polymorphic sequences, acting as a window onto structural transitions. Peptides with conformationally polymorphic silk-like sequences, expected to exhibit an intractable beta-sheet form, were characterized using Fourier transform infrared spectroscopy, circular dichroism, and electron diffraction. Polymorphs resembling the silk I, silk II (beta-sheet), and silk III (threefold polyglycine II-like helix) crystal structures were identified for the peptide fibroin C (GAGAGS repetitive sequence). Two peptides based on silk amorphous sequences, fibroin A (GAGAGY) and fibroin V (GDVGGAGATGGS), crystallized as silk I under most conditions. Methanol treatment of fibroin A resulted in a gradual transition from silk I to silk II, with an intermediate state involving a high proportion of beta-turns. Attenuated total reflectance Fourier transform infrared spectroscopy has been used to observe conformational changes as the peptides adsorb from solution onto a hydrophobic surface. Fibroin C has a beta-strand structure in solution but adopts a silk I-like structure upon adsorption, which when dried on the ZnSe crystal contains silk III crystallites.
蛋白质结构转变和β-折叠的形成在体内和体外都是常见问题，并且在诸如蛋白质加工、β-淀粉样蛋白和朊病毒行为等不同领域具有至关重要的意义。蚕丝提供了一个具有良好特征的多态序列“数据库”，可作为观察结构转变的窗口。使用傅里叶变换红外光谱、圆二色性和电子衍射对具有构象多态的类蚕丝序列的肽进行了表征，这些肽预计会呈现出难以处理的β-折叠形式。对于肽丝心蛋白C（GAGAGS重复序列），鉴定出了类似于丝I、丝II（β-折叠）和丝III（三重聚甘氨酸II样螺旋）晶体结构的多晶型物。基于蚕丝无定形序列的两种肽，丝心蛋白A（GAGAGY）和丝心蛋白V（GDVGGAGATGGS），在大多数条件下结晶为丝I。对丝心蛋白A进行甲醇处理导致从丝I逐渐转变为丝II，中间状态涉及高比例的β-转角。衰减全反射傅里叶变换红外光谱已被用于观察肽从溶液吸附到疏水表面时的构象变化。丝心蛋白C在溶液中具有β-链结构，但吸附后会采用类似丝I的结构，当在ZnSe晶体上干燥时，其中含有丝III微晶。
